Residents have described their shock after a teenager was stabbed to death outside a block of flats in west London.
The boy, believed to be 17, was discovered with stab injuries outside apartments on Union Lane, Isleworth, at around 10.35pm on Friday.
Officers carried out first aid and paramedics were called to the scene, but the victim was pronounced dead a short time later.
Police tape sealed off part of the area on Saturday morning, with officers standing guard.
A couple who called the police described finding the bloodied victim on the ground after the fight.
The husband and wife said they heard shouting and looked out of their flat to see the teenager being assaulted by two people.
The pair, neither of whom wished to be named, said one of the perpetrators had his face covered.
The husband, a 33-year-old engineer, said: “It was one guy who was massive with, like, a mask on his face and another small guy. They were kicking him (the victim), but it was dark so it was hard to see exactly what was happening.”
By the time the couple went downstairs to the scene they said the two attackers had left.
His wife, 35, added: “The boy was bleeding from the mouth. The blood was everywhere. He was still alive, there was someone else there trying to help by checking his pulse. He was responding but he couldn’t speak.”
Her husband said: “It’s sad that he died.”
Police said the victim had been in nearby Syon Park with a group of other people before the incident.
Detective Chief Inspector Richard Leonard from the Metropolitan Police said: “A vehicle has pulled up and a group of males have got out. They have chased the victim and attacked him in Union Lane.”
He appealed for anyone who was in the park or surrounding area at around 10.30pm and who has information that could help to get in contact with the force.
London Mayor Sadiq Khan tweeted: “This is an appalling, senseless act of violence. The family and friends of this young man are in my thoughts at this awful time.”
Halima Abubaker, a 22-year-old hairdresser, said she saw two males “running for their lives” around the time of the incident.
She said: “I just heard loads of people, then there was seven police cars and two vans.
“I’ve lived here two years and nothing like this has happened. I feel really sick.”
Another man, who has lived in one of the flats for seven years, said it was not something he expected to happen near his home.
The restaurant owner and chef, who did not want to give his name, said: “It’s a nice area, it’s nice and quiet. We’ve never had a problem here before.”
No arrests have been made.
Police said the remain open-minded about the motive for the killing.
The victim’s next of kin have been informed but he has not yet been formally identified.
Scotland Yard has launched a murder investigation and is asking anyone with information to contact officers on 101 or Crimestoppers anonymously on 0800 555111.